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Saul is offline
Aug13-10, 02:32 PM
P: 272
This is an interesting subject. There have been discovered very large (up to around 100 kpc) hot (10^7 K) intergalactic gas clouds within galaxy clusters.

A simple calculation based on the density of the gas clouds in question and the emission rate of the hot gas shows the gas clouds should have cooled and formed stars/galaxies.

There are two anomalies. What is the source of energy to heat a very large gas cloud evenly. As noted in this paper AGN and gas in flow will heat the gas cloud unevenly. Also in the case of a gas flow hypothesis there is a mass problem as one needs too much infall gas to continually flow into the original gas cloud.

The second and related problem, is why does the gas cloud not cool.

Interesting also is the mass of the intergalactic cluster gas is approximately the same as the mass of the visible matter (stars) in the cluster's galaxies.

X-ray Spectroscopy of Cooling Clusters

Observations show that the X-ray emission from many clusters of galaxies is sharply peaked around the central brightest galaxy. The inferred radiative cooling time of the gas in that peak, where the temperature drops to the center, is much shorter than the age of the cluster, suggesting the existence of a cooling flow there (Fabian et al. 1994). X-ray spectroscopy over the past 5 yr shows that the temperature drop toward the center is limited to about a factor of three. Just when the gas should be cooling most rapidly it appears not to be cooling at all. This is sometimes known as the cooling flow problem. Careful observations show that gently distributed heat is required over a radius of up to 100 kpc to balance radiative cooling in these regions.
5.7 Definition of the Cooling Flow Problem

We now briefly discuss what we believe the cooling flow problem is, and how it might be resolved. Clearly, the problem is quite complex and it is difficult not to see the problem in historical terms. Peterson et al. (2003), for example, discussed a difference between the soft X-ray cooling-flow problem and the mass sink cooling-flow problem. The former refers to the recent discrepancy seen in the soft X-ray spectrum between what was predicted and what was observed. The latter refers to the difficulty in detecting any by-products in cooling clusters from the hypothesized cooling-flow plasma.

These definitions, however, might just categorize our ignorance of the solution to the problem. The major difficulty is that: 1) the cluster plasma loses energy by emitting the very X-rays we detect, 2) efficient and distributed heat sources 36 are difficult to construct, 3) the cluster plasma appears to cool most of the way, but 4) evidence for complete cooling is utterly lacking. The cooling-flow problem as we see it is to understand what happens in the middle of that process. After examining whether cooling flows are ruled out, we discuss many ideas that might alleviate the cooling-flow problem.

7 Heating
Some heating is always expected in the central regions of clusters. Examples are supernovae (Silk et al. 1986; Domainko et al. 2004), an active central nucleus (Bailey 1982; Tucker & Rosner 1983; Pedlar et al. 1990; Tabor & Binney 1993; Binney & Tabor 1995) and many more recent papers cited in Sec 7.2– 7.5), conduction (Takahara & Takahara 1979; Binney & Cowie 1981; Stewart et al. 1984; Friaca 1986; Bertschinger & Meiksin 1986; Rosner & Tucker 1989) and many more recent papers cited in Sec 7.1). A problem with heating the gas is that the cooling rate is proportional to the density squared whereas most heating processes are proportional to volume. This tends to make the gas unstable and means generally that the cooler denser gas will carry on cooling while hotter surrounding gas heats up. The gas appears to cool by about a factor of three and then stop cooling. A mechanism to do that is not obvious, since the gas does not appear to be piling up at the lower temperature. Indeed it seems that the gas temperature profile is ”frozen” and has been so for some Gyrs (Bauer et al. 2005).
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